Added Facebook token exchange method [SDK-1401]

[Widcket]

Changes

Added

• Added a public method tokenExchange(withFacebookSessionAccessToken:profile:scope:audience:) to exchange Facebook's session info access token and profile data for Auth0 credentials.
• Added a default implementation for that method in the Authentication protocol, that sets default values for the optional parameters.
• Added unit tests for the new method.

Changed

• Moved Auth0Authentication private methods into a private extension to avoid a linter warning due to the length of the struct body.

Fixed

• Explicitly passing nil as the scope value is now respected in the Apple token exchange method. This is backwards-compatible, since nil was the previous default value in that method.
• Fixed a linter whitespace warning.

Testing

• This change adds unit test coverage
• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines
• All existing and new tests complete without errors
• All active GitHub checks have passed